Write a ZK-based samza-standalone JobRunner

SAMZA-516 documents the design of a standalone Samza job, which uses ZK for liveness monitoring and partition assignment rather than YARN. The first step to implementing this design is to add a samza-standalone submodule in Samza, which implements a JobRunner that uses ZK to elect a leader for the job (JobCoordinator), and then start a container. When failures occur, the leader should shift partitions within ZK to move partitions off of the dead container. Likewise, when a new container is added, the leader should shift partitions to the new container. The SamzaContainer should remain immutable, and simply be bounced when partitions are added or removed from it.